Seals of the foregoing type are already known, wherein such seals have a moulded plastic body with a peripheral flange, the bottom face of which is disposed opposite the top face of the metal sheet and about the circumference of the opening when the body is fitted thereto; and an annular bead of meltable glue of the HOT MELT type deposited on the bottom face of the peripheral flange and designed to melt when the metal sheet is heated after the seal is fitted in the opening, so that, after cooling, the seal is fixed to the metal sheet by gluing or adhesion of the peripheral flange to the circumference of the opening.
These seals generally have locking blades which afford a temporary fixation, the final fixation being afforded by the meltable glue, after heating and cooling of the metal sheet, for example, in order to fix the paint which has just been applied to it.